Recently, with the rapid development of information and communication technology, a ubiquitous society is arising based on information and communication technology.
In order for information and communication devices to be connected anytime and anywhere, sensors equipped with a computer chip having a communication function need to be installed in all facilities in society. Therefore, the problem of supplying power to these devices and sensors is a new challenge. In addition, as a variety of portable devices, such as music players, including Bluetooth headsets and iPods, as well as mobile phones, has been rapidly increasing, charging batteries has come to require greater time and effort on the part of users. As a way to solve this problem, a wireless power transmission technology has recently attracted attention.
Wireless power transmission (wireless energy transfer) technology is a technology for wirelessly transmitting electrical energy from a transmitter to a receiver using an electromagnetic induction principle. An electric motor or a transformer that uses such an electromagnetic induction principle was already in use in the 1800s, and thereafter, a method of transferring electrical energy by radiating electromagnetic waves, such as radio waves, a laser, high-frequency waves, and microwaves, has also been attempted. Electric toothbrushes and some wireless shavers that are often used are also actually charged with the electromagnetic induction principle.
Wireless energy transfer schemes devised to date may be broadly classified into an electromagnetic induction scheme, an electromagnetic resonance scheme, and an RF transmission scheme using a short wavelength radio frequency.
The electromagnetic induction scheme is a technology that uses a phenomenon in which a magnetic flux, which is generated when two coils are disposed adjacent to each other and current is applied to one coil, causes the other coil to generate an electromotive force. This technology is being rapidly commercialized around small devices such as mobile phones. The magnetic induction scheme enables the transmission of up to several hundred kilowatts (kW) of power and has high efficiency, but the maximum transmission distance thereof is 1 centimeter (cm) or less, and therefore an object to be charged needs to be disposed adjacent to a charger.
The electromagnetic resonance scheme is characterized in that it uses an electric field or a magnetic field, instead of utilizing electromagnetic waves, currents, or the like. The electromagnetic resonance scheme is advantageously safe to other electronic devices and the human body since it is hardly influenced by electromagnetic waves, which may be problematic. However, the electromagnetic resonance scheme is available only at a limited distance and space, and the energy transfer efficiency thereof is somewhat low.
The short wavelength wireless power transmission scheme, simply put, the RF transmission scheme utilizes the fact that energy may be transmitted and received directly in radio-wave form. This technology is an RF wireless power transmission scheme using a rectenna. The term “rectenna” is a portmanteau of “antenna” and “rectifier”, and refers to a device that directly converts RF power into DC power. In other words, the RF scheme is a technology for converting AC radio waves into DC power, and research on commercialization of the RF scheme has been actively conducted as the efficiency thereof has been improved recently.
The wireless power transmission technology may be applied not only to the mobile industry, but also to various other industries such as the IT, railroad, and home appliance industries.
Generally, the direction of an electromagnetic field is reversed between the inside and the outside of the turns of a closed-loop transmission coil, so that there is a charging shadow area near the turns of the closed-loop transmission coil.
When a reception coil of a wireless power reception device is located in the charging shadow area, wireless charging may not be performed normally.
Therefore, conventionally, an attempt has been made to minimize the charging shadow area by disposing the closed-loop transmission coil in the outermost portion of a charge bed.
However, in a wireless charging system to which the above-described conventional method is applied, a charging-capable area formed outside the closed-loop transmission coil may not be used, and it is necessary for a wireless power transmission device to use a shielding material having a size corresponding to the area of the closed-loop transmission coil.
In addition, in the conventional wireless charging system, since the closed-loop transmission coil is disposed in the outermost portion of the charge bed, the length of the used transmission coil may increase, which is problematic.
Hereinafter, a wireless power device having a plurality of transmission coils according to the related art will be described with reference to FIGS. 1a to 1d. 
Reference characters (a) and (b) of FIG. 1a illustrate a wireless power transmitter and a wireless power receiver of the related art. Referring to reference character (a) of FIG. 1a, the wireless power transmitter 11 incorporates therein a transmission coil 13 for wireless power transmission. The wireless power transmitter 11 transmits wireless power to the wireless power receiver 15 via the transmission coil 13. The wireless power transmitter may transmit power to the wireless power receiver 15 through an electromagnetic resonance scheme. Reference character (b) of FIG. 1a illustrates a side view of the wireless power transmitter 11 and the wireless power receiver 15. The wireless power receiver 15 may be spaced apart from the wireless power transmitter 11 by a distance sufficient to receive wireless power through an electromagnetic resonance scheme.
FIG. 1b is a view for explaining a charging-capable area of the transmission coil 13 described above. The transmission coil 13 may be disposed in an outer peripheral portion of the wireless power transmitter 11. The charging-capable area includes a first area 21 and a second area 25. The first area 21 is located outside the transmission coil 13, and the second area 25 is located inside the transmission coil 13. The terms “outside” and “inside” are defined on the basis of the transmission coil.
Here, a non-charging area may include a third area 23 and a fourth area 27. The third area 23 is a non-charging area in which matching of the impedances of the transmission coil 13 and a reception coil (not illustrated) is difficult. The third area 23 includes an outer non-charging area outside the transmission coil 13 and an inner non-charging area inside the transmission coil 13. The fourth area 27, which is a central area inside the transmission coil 13, has very low magnetic coupling capability with the reception coil, and thus has very low power transmission efficiency.
In order to overcome the limits of FIG. 1b, in the related art, the transmission coil 13 may be disposed, as illustrated in FIG. 1c. The transmission coil 13 is configured as one, but forms two rings. These rings may include an inner ring and an outer ring surrounding the outer periphery of the inner ring. The transmission coil 13 provides an improved inner ring area 33 instead of the fourth area 27 as the non-charging area, but a non-charging area 31 may also be generated in the inner ring area. Thus, charging may be interrupted and inconvenience in use may occur.
In order to overcome the problem of FIG. 1c, in the related art, a second transmission coil 41, which is separate from the first transmission coil 13, is disposed in a charging-capable area of the first transmission coil 13, as illustrated in FIG. 1d. Here, the second transmission coil forms an inner ring and the first transmission coil 13 forms an outer ring. The transmitter alternately applies current to the first transmission coil 13 and the second transmission coil 41 to realize a continuous charge area. However, in the case of FIG. 1d, the magnetic coupling between the first transmission coil 13 and the second transmission coil is very high, thus causing large power loss. This is because the second transmission coil 41 is disposed in the area in which a magnetic field is generated by the first transmission coil 13.
Therefore, there is a demand for the introduction of a more advanced wireless power transmission device.